In order to maintain the operational reliability of low-voltage supply system and information system
it is very important to detect the SPD state timely and effectively. The main element of SPD is varistor. So
the state of varistor is important. For detecting the state of varistor
an live detection device used in SPD detection is developed
which solves the automatic detecting problem about SPD detection on-line. Then the design method of hardware and the software of instrument are presented in detail. The result of field measuring shows that the instrument has excellent performance.